1. Field of the Invention
This invention relates, generally, to laser machining. More particularly, it relates to precise removal of thin films in photovoltaic cells.
2. Description of the Prior Art
Galvanometric scanning mirrors and polygon mirrors are well-known.
Conventional methods of removing thin films from photovoltaic cells employ galvanometer based scan mirrors in combination with a scan lens, typically an F-theta lens. Galvanometer scanning mirrors are limited in precision due to the dither of the galvanometer motors. These motors have typical accuracies of less than twenty microradians (20 μrad). A solar panel can be more than five hundred millimeters (500 mm) wide, corresponding to twenty microns (micrometers) (20 μm) for a scan lens having a one thousand millimeter (1000 mm) focal length. The scribe widths in thin film solar cells are less than one hundred microns (100 μm), so it is desirable to have the accuracy of the scribe be less than five percent (5%) of the scribe width. This is not possible with conventional technologies for thin film scribing of large areas.
U.S. Pat. No. 1,735,108 (1929) discloses a combination plano-concave and plano-convex lens pair as an adjustable wedge or prism. U.S. Pat. No. 7,196,831 discloses a counter rotating disk of lenses. The utility of this structure is limited because the lenses add optical power to the system which the design must accommodate. U.S. Pat. No. 5,387,999 discloses a means to control camera shake but has a very limited field of correction and only applies to cameras. U.S. Pat. No. 6,836,364 discloses a rotational means but the fabrication of the optical components is dependent upon selecting glasses with matching densities but with different indices of refraction. This limits the choice of glass and wavelengths to use and may not be suitable for a high power laser scribing application. U.S. Pat. No. 6,320,705 discloses an adjustable wedge that uses a liquid interface for better matching of the optics. A liquid interface cannot be used in high power laser applications and further prohibits high speed scanning. U.S. Pat. No. 4,436,260 discloses an adjustable wedge that uses an air bearing between two optics. The structure lacks utility in high power laser thin film scribing.